C + + copy constructor details

I. What is a copy constructor

First of all, for ordinary types of objects, the replication between them is very simple, for example:

int  - ; int

And the class object is different from ordinary object, the inner structure of class object is generally more complex, there are various member variables.
Let's look at a simple example of a class object copy.

#include <iostream>using namespacestd;classCexample {Private:intA; Public:      //constructor FunctionCexample (intb) {a=b;} //General Functions     　 voidShow () {cout<<a<<Endl; }};intMain () {Cexample A ( -); Cexample B= A;//Note that object initialization here calls the copy constructor, not the assignmentb.show (); return 0;}

Run the program, screen output 100. As can be seen from the running results of the above code, the system allocates memory for object B and completes the copy process with Object A. In the case of a class object, a class object of the same type is done by copying the constructor to complete the entire copy process.

The following example illustrates the working process of a copy constructor.

#include <iostream>using namespacestd;classCexample {Private:    intA; Public:    //constructor FunctionCexample (intb) {a=b;} //copy ConstructorCexample (Constcexample&C) {a=C.A; }    //General Functions    voidShow () {cout<<a<<Endl; }};intMain () {Cexample A ( -); Cexample B= A;//cexample B (A); it's the same.b.show (); return 0;} 

Cexample (const cexample& C) is our custom copy constructor. As can be seen, the copy constructor is a special constructor , and the name of the function must be the same as the class name, and it must be a reference variable of this type.

Two. Call timing for copy constructors

In C + +, the following three types of objects need to call the copy constructor!
1. Object passed in the function parameter in the way of value passing

classCexample {Private: intA; Public: //constructor FunctionCexample (intb) {a=b; cout<<"creat:"<<a<<Endl;} //Copy ConstructionCexample (Constcexample&C) {a=C.A; cout<<"Copy"<<Endl;} // Destructors~cexample () {cout<<"Delete:"<<a<<Endl;} voidShow () {cout<<a<<Endl; }};//global function, passing in an objectvoidG_fun (cexample C) {cout<<"Test"<<Endl;}intMain () {cexample Test (1); //Incoming Objectg_fun (test);return 0;}

When calling G_fun (), there are several important steps:
(1). When the. Test object passes in the parameter, a temporary variable is generated first, called C.
(2). Then call the copy constructor to give the value of test to C. The whole two steps are a bit like: cexample C (test);
(3). After the execution of the G_fun (), the C object is reconstructed.

2. The object is returned from the function in the way value is passed

classCexample {Private: intA; Public: //constructor FunctionCexample (intb) {a=b;} //Copy ConstructionCexample (Constcexample&C) {a=C.A; cout<<"Copy"<<Endl;} voidShow () {cout<<a<<Endl; }};//Global Functionscexample G_fun () {Cexample temp (0); returntemp;}intMain () {g_fun ();return 0;}

The following important steps occur when the G_fun () function executes to return:
(1). A temporary variable will be generated first, called XXXX Bar.
(2). Then call the copy constructor to give the value of temp to XXXX. The whole two steps are somewhat like: cexample XXXX (temp);
(3). At the end of the function execution, the TEMP local variable is first destructor.
(4). After the execution of the G_fun (), the XXXX object is then reconstructed.

3. Object needs to be initialized with another object

Cexample A (=//

The copy constructor is called in the latter two sentences.

C + + copy constructor details